1. Field of the Invention
The present invention relates to an apparatus for examining gaze shift in the depth direction. More specifically, the present invention relates to an apparatus for examining gaze shift in the depth direction thereby enabling the determination of diseases related to brain functions, such as dementia, by detecting gaze shift of a subject.
2. Description of the Background Art
The number of patients suffering from Alzheimer's disease is estimated to be four million in the United States and about a million in Japan. Compared with senile dementia, such as cerebrovascular disease popular among Japanese, the cause of Alzheimer's disease is not known, and much effort has made to find the cause to enable early diagnosis and early medical treatment. However, it is difficult to discriminate Alzheimer's disease from cerebrovascular disease when there are no typical symptoms. There has been a strong demand of accurate method of discrimination, since the development of the disease and its pharmaceutical treatment are different for these diseases.
Hachinski's ischemic score has been proposed as a method of discriminating these two diseases. According to this ischemic score, a point is given dependent on whether or not the patient has an anamnesis of apoplexy, cerebral infraction or the like, and if the points exceed a prescribed number, it is determined as the cerebrovascular disease, and otherwise it is determined to be Alzheimer's disease. However, discrimination is still difficult by this method if the patient has no such anamnesis.
It has been known that neuropsychological symptom which is considered to be an impairment of "tool function," such as visual cognitive dysfunction, appears from relatively early period of Alzheimer's disease. In view of this fact, Fujii et al. has reported the following analysis carried out by utilizing eye movement. More specifically, a problem of copying a cube on the right side while watching an original of the cube on the left side is presented. Even a patient who is in the initial stage I of Alzheimer's disease and does not show apparent constructional apraxia is reported to show characteristic symptom similar to a so called Balint syndrome; that is, the patient cannot stare at one point, or more specifically, abnormal distribution of gazing point appears, saccade deviated from both the presented cube and the depicted drawing by the patient is generated, or the point of gazing is fixed at the same point for a long period of time. In Alzheimer's disease, it is supposed from MRI (nuclear magnetic periorbital inspection) that there is caused dysfunction of parietal lobe which is related to spatial vision. Accordingly, constructional dysfunction derived from degradation in function of the rear association areas with the parietal lobe being the center, degradation of function of positional recognition of a target point or recognition of depth derived from dysfunction of external spatial vision, such as dysfunction of eye movement, dysfunction of coordinate transformation system between the coordinate of eye movement system and the coordinate of the center of one's body axis, or visual-motor dysfunction, is supposed to be a possible cause of the aforementioned symptoms.